The present invention relates generally to temperature sensing, and more particularly to linear sensing elements for localized overheat detection.
A variety of linear systems exist to sense temperatures and temperature changes for fire and overheat detection in a wide range of applications, including industrial and aerospace applications. Linear systems use elongated sensing elements that extend from a logic-capable detector into one or more sensing regions. Although some systems use a separate detector for each sensing element, many systems join a plurality of sensing elements to single detector. Detectors sense temperature along the sensing elements, and compare temperatures and rates of temperature change with preselected or computed thresholds to flag overheat conditions. Detectors that sense overheat temperatures above a designated threshold may trigger an alarm in a variety of ways, such as by signaling a central control or monitoring system, activating lights or alarms, or storing an alarm event in a maintenance log.
Examples of digital linear detectors include digital alarmline, salt detectors, and pneumatic detectors, each with associated advantages and disadvantages. Digital alarmline use twisted pairs of conductors separated by a polymer insulator that softens or melts when subjected to temperatures above a threshold, allowing adjacent conductors to electrically contact one another. Digital alarmline is simple and inexpensive, but cannot reset, and must be replaced after each alarm. Salt detectors utilize eutectic inorganic salts as variable impedance elements that come into conduction to close an electrical contact when exposed to high temperatures. Salt detectors must be continuously AC powered during operation to avoid polarization effects. Some examples of pneumatic detectors use gases that are evolved from a solid core when heated, producing an increase in fluid pressure within the sensing element that is detected with a diaphragm switch or other pressure sensor. The evolved gas is only partially reabsorbed when the solid core of a pneumatic detector cools, causing a gradual decrease in detector sensitivity.
In addition to identifying overheat conditions corresponding to localized hot spots at temperatures greater than a local alarm threshold Tlocal, some temperature sensing systems are required to throw alarms for distributed overheat conditions corresponding to a high average temperature Tdistributed<Tlocal across a wide region. Various analog sensors, including pressure sensors and optical fiber sensors, are commonly used to sense distributed temperatures.